Urinary tract infections (UTIs), caused primarily by uropathogenic Escherichia coli (UPEC), are among the most common bacterial infections in humans and include infections of the bladder (cystitis) and kidneys (pyelonephritis). Community onset of these infections is predominant in women. However, routine and complicated UTI in male patients is neither nonexistent nor benign; infant boys and many elderly men are susceptible. The clarity of our clinical view of UTI in females and the molecular details of UPEC pathogenesis have advanced immensely in recent years, principally because of a female murine model of cystitis. These studies support a paradigm in which UPEC attach to, invade, and replicate within superficial epithelial cells of the bladder to form intracellular bacterial communities (IBCs). Depending on strain type, female mice in this established model either resolve acute cystitis or progress to chronic cystitis, while few develop lasting kidney infection. Technical barriers to infecting male mice have heretofore precluded the extensive study of male UTI in model hosts, resulting in a tremendous knowledge gap regarding host and microbial mechanisms underlying sex discrepancies in this disease. Addressing this gap, we have developed an innovative mini-surgical technique that bypasses simple anatomic differences to permit infection of the urinary tract in male and female model hosts. This significant advance will allow the proposed work to explore the hypothesis that intrinsic differences in epithelial receptivity and/or immune response of the urinary tract underlie sex differences in susceptibility to these infections. Preliminary results from this new murine model of UTI have been incredibly fruitful, demonstrating that male sex in fact predisposes to chronic cystitis, severe pyelonephritis, and renal abscess. This initial finding correlates with human clinical data indicating that mortality from pyelonephritis is increased in men compared with women. Work in this project will delineate sex differences in intracellular pathogenesis, the IBC pathway, and the host inflammatory response; concretely define the development, resolution, and consequences of severe kidney infections, particularly in male mice; and determine how hormonal milieu and/or sex chromosome complement mechanistically influence these phenotypic sex differences. These studies will illuminate sex-based effects on susceptibility and host response to UTI and establish a robust platform for modeling of pyelonephritis and renal abscesses. Completion of this research will identify key mechanisms underlying sex discrepancies in UTI that can be consequently exploited with novel approaches to limit morbidity and mortality of bladder and kidney infections.